The dystrophin–glycoprotein complex (DGC) provides structural support for the sarcolemma, the membrane that encloses myocytes, and genetic defects in the principal DGC components — dystrophin and sarcoglycan — lead to muscular dystrophy and/or cardiomyopathy in humans and animal models. Now, reporting in The Journal of Cell Biology, Shigekawa and colleagues begin to uncover the molecular events involved in myocyte degeneration.

Increased membrane fragility to mechanical stress and permeability to Ca2+ have been implicated in myocyte degeneration, and elevated Ca2+ concentrations ([Ca2+]) have been reported in dystrophic myocytes. So, the authors decided to look for Ca2+-entry mechanisms that could be responsible for pathogenic myocyte degeneration. They began by searching for mammalian homologues of the Drosophila Ca2+-permeable cation channels, which belong to the transient receptor potential channel family, because they are sensitive to physical stimuli.

Shigekawa and colleagues identified the growth-factor-regulated channel (GRC), which had previously been identified as a Ca2+-permeable non-selective cation channel expressed in non-muscle cells. They found that, although the total GRC content was similar to normal, GRC expression was elevated in the peripheral sarcolemma of cardiac and skeletal muscle of BIO14.6 hamsters (deficient in δ-sarcoglycan), and the skeletal muscle of mdx mice (a model for Duchenne muscular dystrophy) and of myopathic patients.

Next, the authors looked at the properties of BIO14.6 and mdx myotubes. They found that GRC expression was increased in the sarcolemma (in normal myotubes GRC is located mostly in the interior), that it could be reduced by a Ca2+-influx inhibitor (Gd3+) and then re-established by Ca2+. As phosphatidylinositol 3-kinase inhibitors didn't inhibit this translocation, it was concluded that Ca2+ was the primary regulator of GRC translocation. In normal myotubes, GRC translocated to the sarcolemma in response to insulin-like growth factor-1 or cyclic stretch in the presence of Ca2+ — responses that were abolished by Gd3+.

In resting BIO14.6 myocytes, Shigekawa and colleagues found that there was a marked increase in Gd3+-sensitive Ca2+ uptake, and acute elevation of external Ca2+ increased the intracellular [Ca2+] — both responses were suppressed by another Ca2+-influx inhibitor. Moreover, cyclic stretch in the presence of Ca2+ led to a high level of creatine kinase (CK) efflux, which is a marker of myocyte damage. Infection with δ-sarcoglycan cDNA corrected these abnormalities, and treatment with GRC antisense cDNA suppressed both the intracellular [Ca2+] increase and the CK efflux.

By expressing GRC on the surface of CHO cells, which lack endogenous GRC, the authors also showed that Ca2+ influx through GRC was increased in the absence of stretch, that stretch enhanced the influx and that it caused cytoskeletal reorganization. And finally, from results obtained with cardiac-specific GRC transgenic mice, “it seems likely that elevated levels of sarcolemmal GRC result in greater Ca2+ influx in response to mechanical stress in cardiac chamber walls, causing further mobilization of GRC on the cell surface, thereby exacerbating Ca2+ overloading and the resultant cell damage”.

So, the results obtained by Shigekawa and colleagues “...suggest that GRC is a key player in the pathogenesis of myocyte degeneration caused by dystrophin–glycoprotein complex disruption”.